Remove 'minordefault' preference completely

Remove 'minordefault' preference completely

See bug 24313.  The preference encourages people to mark changes minor
by mistake, but it's better to have minor changes not marked minor than
non-minor changes marked minor.  enwiki users at [[Help talk:Minor
edit]] seemed to think it was a good idea.  Plus, I'm always in favor of
removing user preferences where possible.

The only likely fallout of note is some bot operators who didn't notice
this suddenly find their bot not marking anything minor.  I doubt it
will cause major problems, though.